The paint, ceramic, electronic and other industries utilize suspensions of particles in liquids as a stock material in the production of various end products, such as paints, ceramics, and electrical products. In many cases, these stock particle suspensions are stored without agitation and the particles, being of a greater density than the liquid, settle out due to the density mismatch and eventually form a hard sediment layer at the bottom of the storage container. This is undesirable since the particles comprising the sediment layer are generally difficult to redisperse into suspension to prepare the stock solution for use. It is therefore advantageous to have a particle suspension that is thixotropic so that for working purposes it is fluid during and immediately after shearing but when quiescent as during storage it will eventually and reversibly set up like a gel prior to significant sedimentation of its particles.
Some particle suspensions are naturally thixotropic. For example, particles that are of an anisotropic shape, i.e., of a plate or needle-like shape, generally form thixotropic suspensions. Further, with respect to particles suspended in a polar medium such as water, it is known that certain additives, such as salts or colloidal silica, can be added to render them thixotropic. In some cases these thixotropic inducing additives are otherwise undesirable, for example, they are undesirable in a ceramic sheet forming particle suspension since the additive cannot be removed by heating prior to use of the suspension. Hence, the thixotropic inducing additive becomes a part of the final product and in this context it may be considered to be an undesired contaminant.
Many end uses require the use of particles whose shape is not unique in nature and which will not therefore naturally result in a thixotropic suspension. With respect to such particles, they may be prepared as a thixotropic particle suspension only in an aqueous medium and then only by addition of a thixotropic inducing additive to the medium which, in many cases, may be an undesirable component in the final product to be produced from the particle suspension. Further, when the end use application requires the medium to be a non-polar liquid, such as is the case with an electrorheological fluid, it is even more difficult to prepare a suspension of equiaxed particles which is thixotropic.
Another approach which has been taken to reduce the rate of sedimentation of particles from a particle suspension which cannot be prepared in thixotropic form is to make the suspension shear thinning, meaning it has a higher viscosity when quiescent, but under application of shear, the viscosity decreases. Although this shear thinning approach may retard the rate at which sedimentation occurs, it does not totally prevent the occurrence of sedimentation or to be effective to inhibit particle settlement the suspension may have a thinned viscosity above that desirable for working purposes of the suspension.
To inhibit gravitational sedimentation of their particles during quiescent storage it is still a goal in this art to develop a method for forming particle suspensions that are thixotropic, irrespective of the shape of the particles, which is independent of the nature of the suspending liquid as being polar or non-polar, and which does not require the presence of an extrinsic thixotropic inducing additive.